The literature on the solid state stability of drugs and natural products has been reviewed and three solid state reactions have been chosen for careful study. These reactions were chosen because each could be of significance to synthetic, mechanistic, and applied chemistry. The solid state reactions of Vitamin D2 (calciferol), acetyl salicyclic acid anhydride and L-1,4-cyclohexadiene-1-alanine are of broad interest because reports indicate that these compounds crystallize in one form which is reactive and in another form (or forms) which is less reactive. These reactions are important synthetically because of the ability to control reactivity by selecting which crystal form is present and because there are probably differences in the reaction products of the solid state and solution reactions. They are mechanistically important because the different environments of the molecules in the reactive and unreactive solids must be responsible for their differences in reactivity. An understanding of these environmental factors should indicate the steric requirements of these reactions. The results of studies of these reactions could be directly applied to developing new methods of stabilizing drugs in the solid state.